It is sometimes desired to provide a tire with a rubber tread to promote reduced rolling resistance for the tire itself and thereby improved fuel economy for an associated vehicle, as well as reduced heat buildup in the tire tread during operation of the tire which, in turn, is expected to promote improved tire tread durability.
To promote such one or more of such desirable properties, it is sometimes desired to promote a reduction in the hysteretic property of the tread rubber.
To promote a reduction in the hysteresis of the tread rubber (e.g. to promote a reduction in the rubber's physical rebound property) it may desired to significantly reduce its reinforcing carbon black content with an attendant increase its precipitated silica content.
The rubber reinforcing carbon black content of the tread rubber may be reduced, for example, to less than 30 phr or even less than 20 phr or even reduced to a maximum of 10 phr which, in turn promotes a significant increase in electrical resistivity (decrease in electrical conductivity) of the rubber composition. This may significantly increase electrical resistance to passage of static electrical electricity (thereby promoting a buildup of static electricity) between the tire and the running surface (the ground-contacting surface) of the tire tread, particularly as the carbon black content falls below what is known as its percolation point. Such phenomenon is well known to those having skill in such art.
It would therefore be advantageous to provide a tread rubber composition with significantly reduced rubber reinforcing carbon black content but, nevertheless, to promote an improvement in its electrical conductivity.
For this invention it is proposed to provide an inclusion of significantly electrically conductive carbon nanotubes, particularly functionalized carbon nanotubes, in the tread rubber composition to aid in discharging static electricity through the electrically non-conductive tread to its running surface.
Carbon nanotubes have heretofore been suggested for inclusion in rubber compositions, including tire treads, for various purposes. For example, and not intended to be limiting, see Patent Publications: U.S. Pat. No. 6,476,154, US2006/0061011, US2010/0078194, US2011/0146859, WO2003/060002, DE 102007056689, JP2009/046547, KR 100635604 and KR 2005027415.
In one embodiment, use of functionalized carbon nanotubes in a diene-based elastomer containing rubber composition which contains particulate reinforcement in a form of combination of rubber reinforcing carbon black and precipitated silica (amorphous synthetic precipitated silica which hydroxyl groups on its surface) together with silica coupler for the precipitated silica is contemplated for said tread rubber composition.
The carbon nanotubes are conventionally nano-sized particles in a sense of having an average diameter in a range of from about 1 nm to about 100 nm and an average L/D (length to diameter dimension, or ratio) in a range of from about 10/1 to about 10,000/1.
Such carbon nanotubes are conventionally prepared by, for example, by passing a gaseous carbon-containing compound such as for example, at least one of acetylene and ethanol, usually contained in nitrogen or hydrogen through or over a heated catalyst (e.g. heated to about 700° C.) of metal nanoparticles. Carbon deposited on the metallic nanoparticles is a form of the carbon nanotubes is recovered.
One important aspect of this invention is to provide the carbon nanotubes in a functionalized form to promote an electrically conductive property to the rubber composition and to rely on the precipitated silica, together with a coupling agent for the precipitated silica, without an appreciable rubber reinforcing carbon black to provide reinforcement for the rubber composition. The functionalization of the carbon nanotubes is envisioned as promoting their dispersion within the rubber composition and to thereby promote rubber-to-filler (the nanotube filler reinforcement) within the rubber composition to promote low rolling resistance and treadwear resistance for a tire tread rubber composition as well as to promote electrical conductivity for the rubber composition and thereby the tire tread.
It is believed that a tire with a tread which contains a minimal, if any, rubber reinforcing carbon black together with having a significant functionalized carbon nanotube content is a departure from past practice.
It is envisioned, in one embodiment, that the carbon nanotubes may be coupled to the diene-based elastomer(s) of the rubber composition by a nanotube coupling agent to thereby rely on pi-pi (π-π) bonding of the carbon nanotubes to the diene-based elastomer(s) and sulfur bonding to, or sulfur co-curing with, the diene based elastomer(s). There the nanotube coupling agent has a moiety with pi electron configuration to bond to the carbon nanotubes also having a pi electron configuration to from a pi-pi network and another different moiety comprised of a polysulfide moiety to interact with carbon-to-carbon double bonds of the diene-based elastomer(s) of the rubber composition or comprised of a carbon-to-carbon double bond to sulfur co-cure with the diene-based elastomer(s).
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.